In the case of known torque-limiting, power operated, screw tightening tools, torque is transferred from the driving shaft via a connecting coupling to the driven shaft. The driven side of the connecting coupling is coupled to the driven shaft by means of a ball and groove system. Specifically, one or more balls simultaneously engage specially shaped grooves and/or pockets in the connecting coupling. When such known tools are used for tightening a screw, the balls, move in the grooves as torque increases. Because the grooves rise in the direction of rotation, the driven side of the connecting coupling is, displaced axially toward its disconnect position as torque increases. The connecting coupling is actuated at a preset reaction torque to disconnect the drive. If the connecting coupling is not disengaged as close as possible to the point in time when the preset reaction torque is reached, torque may still be transferred to the driven shaft. This remaining torque may continue turning the fastener so that the desired elastic region is exceeded and the fastener is deformed in a plastic manner.
In such prior art tools, claws on both sides of a connecting coupling bear against each other with increasingly smaller surfaces. Consequently, the claws of the connecting coupling experience increased wear. In addition, even after the driven parts are disconnected from the driver, the torque still affects the driven shaft so that the driven shaft may be exposed to uncontrolled movements.
The task of the present invention is to provide a torque-limiting, power operated, screw tightening tool which, upon attainment of a predetermined torque, disconnects transfer of the torque to the driven shaft completely thus avoiding plastic deformation of the fastener by transfer of residual or remaining torque to the fastener after the predetermined level of torque has been reached.